It is known from J. Am. Ceram. Soc., 68 (5)273-78 (1985) that zirconyl phosphate cannot be sintered to materials of significant strength without additives.
In addition, it is known from DE-A No. 3 609 730 that powder mixtures capable of compaction to ceramics of low thermal expansion and high strength by sintering at 1200.degree. to 1700.degree. C. can be produced from mixtures of zirconyl phosphate with compaction aids, such as magnesium oxide, zinc oxide, bismuth oxide, manganese oxide, iron oxide, cobalt oxide, nickel oxide, titanium oxide, cerium oxide, niobium oxide, tantalum oxide or mixtures of these oxides.
In addition, it is known that additions of grain growth promoters, such as SiO.sub.2 or metal silicates, produce favorable results in conjunction with the known compaction aids.
One feature common to all these mixtures is that melts of metal orthophosphates are formed during the sintering process, enabling the zirconyl phosphate to be compacted. The ceramics formed are thus all multiple-phase ceramics which means that in addition to .alpha.-zirconyl phosphate, other substances are present in the materials in quantities of more than 1%.
The disadvantage of mixtures such as these is that irreversible property changes occur during long-term use at elevated temperature or even at in-use temperatures above the production temperature. This is all the less the case, the less the volume of melt phase which occurs during heating.
Accordingly, the object of the present invention is to provide zirconyl phosphate ceramics which do not have any of the decribed disadvantages.